The present disclosure is related to underfill materials utilized in electronic devices. More particularly, the present disclosure is related to novel no-flow underfill compositions including an epoxy resin in combination with epoxy hardener, and optional reagents. The epoxy hardener can be a difunctional siloxane anhydride or a mixture of a difunctional siloxane anhydride with a liquid organic anhydride. The resulting underfill material has enhanced adhesion and improved physical properties.
Demand for smaller and more sophisticated electronic devices continues to drive the electronic industry towards improved integrated circuit packages that are capable of supporting higher input/output (I/O) density as well as possessing enhanced performance with smaller die areas. While flip chip technology has been utilized to respond to these demanding requirements, a weak point of the flip chip construction is the significant mechanical stress experienced by solder bumps during thermal cycling. This stress is due to the coefficient of thermal expansion (CTE) mismatch between silicon die and substrate that, in turn, causes mechanical and electrical failures of the electronic devices.
Currently, capillary underfill is used to fill gaps between the silicon chip and substrate and improves the fatigue life of solder bumps. Unfortunately, many encapsulant compounds utilized in such underfill materials suffer from the inability to fill small gaps (50-100 μm) between the chip and substrate due to high filler content and high viscosity of the encapsulant.
While a new process, no-flow underfill, has been developed to address these issues, the use of resins filled with conventional fillers in these processes remains problematic. In the case of the no-flow process, application of the underfill resin is performed before die placement, a process change that avoids the time delay associated with wicking of the material under the die. In some cases, there is poor adhesion of the underfill materials to the chip and substrate and cracking can occur during thermocycling. In no-flow underfill applications, it is also desirable to avoid entrapment of filler particles during solder joint formulation. Thus, there remains a need to find a material that has a high glass transition temperature, low coefficient of thermal expansion and ability to form reliable solder joints during a reflow process such that it can fill small gaps between chips and substrates.